Backend: add missing unit tests for untested components

[pcastelo]

Problem

Several critical backend components have zero test coverage.

Missing test classes (priority order)

High priority (security + core functionality)

• ChatResource / ChatService — AI interaction, prompt injection vectors
• ClusterService — cluster CRUD, token management
• ThreeScaleResource — REST layer for 3scale operations
• MigrationResource — REST layer validation (currently only service is tested)
• AuditService / AuditResource — audit trail integrity

Medium priority (generators + tools)

• RateLimitResourceGenerator — rate limit policy YAML generation
• AuthPolicyResourceGenerator — auth policy YAML generation
• PlanPolicyResourceGenerator — plan policy YAML generation
• KuadrantCtlService — external CLI invocation
• ExportManifestValidator — manifest validation logic

Lower priority (MCP + metrics)

• ConnectivityLinkMcpTools — MCP tool execution
• MigrationAgent / GateForgeTools — AI agent tools
• GateForgeMetrics — metrics registration
• HubResource — Developer Hub integration

Acceptance criteria

• At least one unit test per class above
• Test both happy path and error scenarios
• Use mock/stub for external dependencies (K8s client, AI, 3scale API)
• All new tests pass in CI